In a known electric assist bicycle including a motor, power is fed from a power storage such as a battery, a human driving force, which includes a pedal force applied to a pedal, is detected by a torque sensor, and an auxiliary driving force (assisting force) of the motor is added according to the human driving force. Thus, such an electric assist bicycle can smoothly travel on an uphill or the like.
In such an electric assist bicycle, a motor drive unit including the motor is disposed at a crank shaft. Moreover, the electric assist bicycle configured thus has a relatively heavy motor drive unit that is disposed at a low position at the center of the electric assist bicycle (that is, the intermediate point between the front wheel and the rear wheel) in the longitudinal direction thereof. Thus, the front and rear wheels of the electric assist bicycle configured thus can be more easily lifted than an electric assist bicycle having a motor in the hub of the front or rear wheel. Such an electric assist bicycle can easily pass over a step of a path, achieving ease of handling and high riding stability.
Motor drive units to be provided in such an electric assist bicycle are broadly classified into a so-called double-shaft motor drive unit 100 that includes, as shown in FIG. 14, an auxiliary-driving force output sprocket 103 that outputs an auxiliary driving force from a motor in addition to a driving sprocket (also called a front sprocket or a chain sprocket) 102 serving as a human driving force output wheel disposed near one end of a crank shaft 101, and a so-called single-shaft motor drive unit 200, as shown in FIGS. 15 and 16, in which a human driving force generated by a pedal force and an auxiliary driving force generated by a motor are combined in the motor drive unit 200 and the combined force is outputted from a driving sprocket 201.
The double-shaft motor drive unit 100 is disclosed in, for example, Patent Literature 1. As shown in FIG. 14, the auxiliary-driving force output sprocket 103 protrudes to the outside of a unit case 104 of the motor drive unit 100 from a portion behind the driving sprocket 102 in the motor drive unit 100. The driving sprocket 102 that outputs a human driving force and the auxiliary-driving force output sprocket 103 that outputs an auxiliary driving force are engaged with a chain 105 serving as an endless driving force transmission member. The human driving force and the auxiliary driving force are combined by the chain 105 and then the combined force is transmitted to the rear wheel.
Further behind the auxiliary-driving force output sprocket 103, a tensioner device (also called a guide device) 106 is provided in engagement with the chain 105, which has been engaged with the auxiliary-driving force output sprocket 103, so as to guide the chain 105 downward. Moreover, a tension sprocket 107 provided in the tensioner device 106 increases the winding angle of the chain 105 engaged with the auxiliary-driving force output sprocket 103.
Meanwhile, the so-called single-shaft motor drive unit 200 is disclosed in, for example, Patent Literature 2. As shown in FIGS. 15 and 16, the outer periphery of a crank shaft 202 that receives a human driving force transmitted from the pedal has a cylindrical human-power transmission member 203 that receives the human driving force transmitted by serration coupling and so on, and a combined force member 205 where the human driving force transmitted via the human-power transmission member 203 is combined with an auxiliary driving force from a motor 204. Moreover, the human driving force from the human-power transmission member 203 is transmitted to the combined force member 205 via a one-way clutch 206. A large-diameter gear 205a that receives an auxiliary driving force from the motor 204 via a speed reduction mechanism 207 is formed on one end of the combined force member 205, whereas the driving sprocket 201 is attached to another end of the combined force member 205, the driving sprocket 201 serving as a driving force output wheel engaged with a chain 208 serving as an endless driving force transmission member. A combined force on the combined force member 205 is transmitted from the driving sprocket 201 to the rear wheel through the chain 208.
As shown in FIGS. 15 and 16, the single-shaft motor drive unit 200 is configured such that only the driving sprocket 201 is engaged with the chain 208 and the combined force of a human driving force and an auxiliary driving force is transmitted to the chain 208. In contrast, the double-shaft motor drive unit 100 needs to engage, as shown in FIG. 14, the driving sprocket 102 for transmitting a human driving force, the auxiliary-driving force output sprocket 103 for transmitting an auxiliary driving force, and the tension sprocket 107 with the chain 105.
Thus, the area of the single-shaft motor drive unit 200 in side view (laterally projected area) can be advantageously smaller (can be made compact) than that of the double-shaft motor drive unit 100 by devising the layout of the motor 204 and the speed reduction mechanism 207. A so-called front derailleur can be attached to the single-shaft motor drive unit 200 including the driving sprocket 201 with multiple stages. On the other hand in the double-shaft motor drive unit 100, it is necessary to engage the driving sprocket 102, the auxiliary-driving force output sprocket 103, and the tension sprocket 107 with the chain 105, leading to difficulty in attaching the front derailleur.
Moreover, the single-shaft motor drive unit 200 advantageously eliminates the need for providing the tensioner device 106 of the tension sprocket 107 or the like. Generally, braking devices used for electric assist bicycles include a rim brake, a band brake, and a roller brake that are operated with a brake lever attached to a handle bar as those of ordinary bicycles. Depending on the regions or the request of an operator, the attachment of a coaster brake to the rear wheel may be required. The coaster brake is operated by rotating the pedals opposite to a forward rotation direction. In this case, the pedals rotated in the opposite direction apply a tension that pulls the lower part of the chain forward. Thus, the double-shaft motor drive unit 100 needs a unique design for the tensioner device 106, whereas the single-shaft motor drive unit 200 advantageously eliminates the need for such a unique design.
Typically, in the foregoing advantageous single-shaft motor drive unit 200, a magneto-striction torque sensor 209 for detecting a human driving force is provided on the outer periphery of the human-power transmission member 203, which receives a human driving force transmitted from the crank shaft 202, and a portion opposed to the outer periphery. Specifically, a magneto-striction generation portion 209b is formed on the outer periphery of the human-power transmission member 203, and a coil 209a for detecting a change of magnetism on the magneto-striction generation portion 209b is disposed to oppose to the magneto-striction generation portion 209b. When the right and left pedals are pressed, the crank shaft 202 is twisted by a pedal force (human driving force). Thus, the twisted state of the human-power transmission member 203 that receives a human driving force transmitted from the crank shaft 202 is detected by the torque sensor 209.
If a front derailleur is attached to the single-shaft motor drive unit 200, an external derailleur for a sport bicycle or the like may be provided on the side of the crank shaft. Specifically, a plurality of driving sprockets may be disposed in the location of the driving sprocket 201 shown in FIG. 16 so as to be displaced from each other along the axial direction of the crank shaft 202 (also referred to as a vehicle width direction) and a front derailleur that moves the chain in the vehicle width direction may be provided near the driving sprockets.